The proposed studies are of broad importance and impact because developmental defects in the heart can lead to problems in embryogenesis but also problems that are manifested in childhood or even adult life. Control of cardiac myocyte division, shape and vesicular transport is critical for cardiac morphogenesis. While regulation of these events by a single gene may seem improbable, our recently published studies demonstrate that Lek1 plays critical roles in regulation of cell division, shape and transport. The major criticism of the original application was that we lacked data supporting Lek1 function in cardiac myocytes and that inhibition of Lek1 function was not linked to any developmental defect or cardiac disease. We now demonstrate that 1) Lek1 regulates these critical functions in cardiac myocytes and that 2) conditional disruption of the Lek1 gene results in developmental abnormalities of the heart wall leading to dilated cardiomyopathy. These new data lead us to the central hypothesis that Lek1 plays a central role in cardiac morphogenesis and that mutation of this gene leads to impaired function in the differentiated heart. Three straightforward aims will test this hypothesis. The first aim will determine whether changes in cardiac myocyte division, growth and/or apoptosis account for the "small heart" phenotype. This will be accompanied by physiological analyses of the postnatal heart to determine whether the normal adaptation of the myocyte is disrupted. The second aim will quantify the degree and timing of vesicular transport disruption in the developing and postnatal heart using GLUT4 trafficking as a model. The second part of this aim is more speculative but simple in terms of its analysis. Namely, we will determine how generation and maintenance of the intercalated disc is altered with mutation of the Lek1 gene. This will not only serve as a model for vesicular transport and myocyte shape but provide critical insight into regulation of the disc which is essential for cardiac function. The third aim focuses on determining the specific roles of nuc- and cyt-LEK1 in heart development. This will be accomplished using transgenic "rescue" of the Lek1-/- heart. No other group has the genetic, molecular, immunochemical or cell biological tools needed to elucidate the function of this unique gene in cardiac development and disease. [unreadable] [unreadable] [unreadable]